Feature Request: display available memory instead of used+buffers/cache

[trumbaut]

Currently, memory usage is shown based on used + buffers/cache counters (https://github.com/OutsideIT/FireMotD/blob/master/FireMotD#L401), for example:

Memory -> Free: 0.22GB (1%), Used: 31.03GB (99%), Total: 31.26GB

This is confusing for less experienced users (cfr. https://www.linuxatemyram.com/). It might be useful to display available memory instead/too, based on the available parameter of the free command:

$ free -g
              total        used        free      shared  buff/cache   available
Mem:             31          16           0           0          14          13
Swap:             1           0           1